Tuesday, August 31, 2010
Control Your Depth of Field
Try the Focus Slice Auto-Align technique to get tack-sharpness from near to far
Helicon Focus was initially designed for microscopists. The idea is to take photographs at many focusing points through a scene to produce a final blended image that’s sharp throughout. While the technique works well, and it’s also useful for the landscape photographer, the technique has some inherent limitations. Helicon Focus is an interpolation technique that can produce some problems such as “ringing artifacts.” These are rings around some structures, especially sharp-edged, bright subjects such as trillium flowers.
Also, if there’s movement in the image (water, moving clouds), serious image degradation occurs. The Helicon Focus designers recognized both problems and have designed a Retouching tool to deal with them; however, I’ve had limited success with retouching.
Focus Slice Auto-Align
Another technique for precisely controlling sharp depth of field uses “focus slicing” as you would do for Helicon Focus, but the images are combined using Auto-Align in Photoshop. The Focus Slice Auto-Align technique has no inherent limitations because the procedure isn’t interpolative, and the focus slices are layered on each other with the sharp zones of each slice “painted” onto the final composite image.
The lens limitations are concerned with diffraction problems caused by small-aperture diameters at larger ƒ-stops in SLR lenses. ƒ-stop is defined by the mathematical equation: ƒ-stop = lens focal length divided by aperture diameter.
For a wide-angle lens such as a 24mm at ƒ/22, the aperture diameter is approximately 1mm. That’s a small opening. So when the lens focuses an image and it passes through an aperture at ƒ/22, there’s considerable diffraction as the image passes through the small aperture and is focused on the sensor (or film). At apertures ƒ/16 and beyond, there’s a progressive degradation in image quality, so SLR lenses should be used at “sweet spots,” ƒ-stops around ƒ/8 to ƒ/11, when possible. Of course, that means a reduction in depth of field, so to use focus-slicing techniques, it’s best to use several focus slices to ensure sharpness throughout the final composite.
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